Holes in the ozone layer caused by burning space debris

In 2014, 90 rockets were launched into Earth’s orbit; in 2024, that number had risen to 258. Those rockets carried 2,849 objects into space, primarily satellites — more than ten times as many as in 2014. This spectacular growth is largely due to the rise of so-called megaconstellations: networks of thousands of satellites, such as SpaceX’s Starlink. Projections suggest that by 2035, some one hundred thousand satellites will be circling the Earth, compared to just over eleven thousand today.

Catalyst

Almost all of these satellites and their associated rocket stages eventually fall back to Earth at speeds of thousands of kilometres per hour, burning up high in the atmosphere. Until recently, such a reentry was considered a straightforward solution. But with numbers like these, that is no longer automatically the case, warned José Ferreira of the University of California, Los Angeles (UCLA), and colleagues in an article published in late 2024 in Geophysical Research Letters.

‘Reentry scenarios for megaconstellations point to 360 tonnes of aluminium oxide per year; that could significantly damage the ozone layer’

They conducted a simulation of the reentry of a typical 250-kilogram satellite, which is largely composed of aluminium. Such a reentry introduces approximately 30 kilograms of aluminium oxide nanoparticles — or alumina — into the atmosphere. One of the potential consequences concerns the ozone layer, which protects life on Earth from harmful ultraviolet radiation. Alumina acts as a catalyst for the reaction between chlorine and ozone. When researchers discovered in the 1980s that chlorofluorocarbons released from refrigeration systems were damaging the ozone layer, measures were taken, and the ozone hole has been slowly recovering ever since.

‘But reentry scenarios for megaconstellations point to 360 tonnes of aluminium oxide per year, which could lead to significant ozone depletion,’ the researchers write. They also note that alumina particles absorb sunlight. This leads to warming of the upper atmosphere, which would in turn have a cooling effect on the lower atmospheric layers. However, the particles could also reflect radiation re-emitted by the Earth, a greenhouse effect that would drive warming. Here too, the balance of effects is still poorly understood, says Ferreira.

Measurement campaign

It is not in itself surprising that burning satellites produce aluminium oxide, says Ferreira, but the size of the combustion products was. In their molecular dynamics simulation, the researchers accounted not only for the chemical reaction between aluminium and oxygen, but also for the physical effects of the extreme speeds involved in a reentry. Ferreira: ‘We expected the particles to be micrometres in size, but the research indicates we are in the nanometre regime.’ As a result, the particles have a relatively larger reactive surface area and fall more slowly. They can therefore remain suspended in the atmosphere for decades.

‘The balance of effects is still poorly understood’

José Ferreira (UCLA)

Moreover, it is not only simulations but also measurements that point to a growing presence of spacecraft residues in the upper atmosphere. Daniel Murphy of the American National Oceanic and Atmospheric Administration (NOAA) and colleagues published the results of a high-altitude aircraft measurement campaign in PNAS. Ten per cent of the sulphuric acid particles in the atmosphere — which occur there naturally as a result of volcanism — contained metals such as aluminium, copper, and lithium, in roughly the same proportions in which they are used in satellites. Reentries are therefore already leaving their mark on the composition of the upper atmospheric layers.

Warming effect

In addition, soot and nitrogen oxides from the space industry also affect atmospheric chemistry, researchers from University College London calculated in a paper published in Earth’s Future. Soot is produced during launches through the combustion of rocket fuels such as kerosene and methane, but also during reentries. Although the quantities are far smaller than those from terrestrial sources, the fact that they are deposited in the upper atmospheric layers means the warming effect can be up to 500 times greater. Furthermore, reentries also generate nitrogen oxide: the extreme temperatures cause normally inert nitrogen and oxygen in the air to react with one another, forming nitric oxide, which can in turn deplete the ozone layer.

The effect of ozone depletion has so far been modest, says Connor Barker, one of the UCL researchers. ‘In 2022, terrestrial sources caused 2 per cent ozone depletion, and of that, only 0.1 per cent is attributable to the space industry,’ says Barker — just one twentieth. But that share will almost certainly grow. Barker: ‘There is actually a great deal of overlap with geoengineering, where we deliberately introduce particles into the atmosphere to influence the climate.’ According to Ferreira, in addition to further research, regulation of the space industry will ultimately be needed. ‘We are not saying this is going to destroy the ozone layer. We simply don’t know — but we do know that we need to pay attention to this.’